Hours after the bridge collapsed, a city official said early Friday that the rescue mission is now a search and recovery effort.

At least nine people were transported to hospitals for treatment, authorities said.

Witnesses described hearing a loud boom and moments later, finding victims, including construction workers across the wreckage and people trapped in cars.

Senator Marco Rubio, who visited the site Thursday, posted on Twitter: "The cables that suspend the #Miami bridge had loosened & the engineering firm ordered that they be tightened. They were being tightened when it collapsed today." Rubio has been a visiting professor at the university for the past 10 years.

The bridge's span had just been installed Saturday near the university in Miami.

Latest developments

A drone captures the scene where a pedestrian bridge collapsed at Florida International University in Miami.CNN

• Four deceased victims were located but have not been identified, Miami-Dade County Fire Chief Dave Downey.

• Florida Gov. Rick Scott said the state will determine whether any wrongdoing led to the collapse and, if so, people would be held accountable.

• FIU president Mark Rosenberg said the university followed all the processes required during the building of the bridge, and all contractors are certified by the state.

Witness: 'Sounded like the world was ending'

Witnesses described a confusing and horrifying scene.

Giovanni Hernandez said it "sounded like a bomb, like multiple bombs in one."

"It sounded like the world was ending, and when you look back, all you see is the bridge on the floor. It was awful," he told CNN affiliate WSVN.

Sweetwater Police Sgt. Jenna Mendez was stopped at a red light one intersection away when she saw the bridge crumple before her eyes. She told CNN affiliate WINK that she initially thought, "Why are they taking the bridge down?"

After seeing the collapse, she sprang into action and found four injured construction workers.

"They were working on top," Mendez told CNN affiliate WPLG. "They fell several feet obviously. We had one that wasn't breathing. We had one that had laceration to his head that was unconscious, and we had two that were in major shock."

Doctors and medical students ran to the scene from a nearby building and started giving medical attention to victims, said Isabella Carrasco, a student at the University of Miami, who had just passed underneath the bridge in a car.

Carrasco said she saw at least five or six cars crushed beneath the bridge.

"Someone on the side of the road had asked a police officer if she had heard any response from the people inside the car," Carrasco said, "and she shook her head and said no."

Bridge was still under construction

The bridge, which was under construction, was meant to connect the FIU campus to the Sweetwater neighborhood, home to more than 4,000 of its students, according to a news release on the school's website. FIU is the second largest public university in Florida with about 50,000 students.

The bridge was scheduled to open to foot traffic and cyclists in 2019, and boost safety on a busy street where an FIU student was fatally struck by a vehicle last year.

"It is exactly the opposite of what we had intended and we want to express our deepest condolences to the family and loved ones of those who have been affected," FIU President Mark Rosenberg said in a video.

"The bridge was about collaboration, about neighborliness, about doing the right thing but today we're sad. And all we can do is promise a very thorough investigation, to getting to the bottom of this and mourn those who we have lost."

The bridge was designed to withstand the strength of a Category 5 hurricane, according to a fact sheet on FIU's website, and was supposed to last for more than 100 years. It cost $14.2 million to build and was funded as part of a $19.4 million grant from the US Department of Transportation.

Accelerated Bridge Construction (ABC) technology was used in the construction, according to the university. ABC streamlines the building process so bridge construction projects can be completed quicker and be more cost-efficient.

Who was building the bridge?

Companies involved in the bridge construction expressed sympathy for the victims and pledged to find out what happened.

MCM, one of the companies, said in a statement it would "conduct a full investigation to determine exactly what went wrong and will cooperate with investigators on scene in every way."

Another company, FIGG Bridge Engineers, which designed the pedestrian bridge stated: "We will fully cooperate with every appropriate authority in reviewing what happened and why. In our 40-year history, nothing like this has ever happened before."

Bolton Perez and Associates, a third company involved in the construction of the bridge, is not commenting at this time.

The National Transportation Safety Board will lead the federal inquiry.